The present invention relates to nonwoven webs of poly(vinyl alcohol) fibers. More particularly, the present invention relates to a method of preparing a nonwoven web of poly(vinyl alcohol) fibers.
Continuous filaments of poly(vinyl alcohol), i.e., poly(vinyl alcohol) textile fibers, in general are prepared by either wet spinning or dry spinning. Wet spinning generally involves extruding an aqueous solution of the polymer into a coagulating bath, such as a solution of sodium sulfate in water. Dry spinning, on the other hand, generally involves extruding an aqueous solution of the polymer into air. In this case, though, the polymer solution typically is highly concentrated and the extruded liquid filaments are solidified, dried, hot-drawn, and heat-treated in a gaseous environment. Wet spinning also has been utilized for the production of filaments from a water-insoluble, thermoplastic polymer, poly(ethylene terephthalate); see U.S. Pat. No. 4,968,471 to Ito et al.
Dry spinning is classified into two types: (a) low-draft spinning and (b) high-draft spinning. The two types differ in the magnitude of the draft which is defined as the ratio of the take-up speed of the filaments to the extrusion speed of the spinning solution from the die. For a general discussion of the dry spinning of poly(vinyl alcohol), see Ichiro Sakurada, "Polyvinyl Alcohol Fibers," Marcel Dekker, Inc., New York, 1985, pp. 249-267.
The basic principles involved in dry spinning have been applied to the formation of nonwoven webs. For example, U.S. Pat. No. 4,855,179 to Bourland et al. describes the production of superabsorbent articles in the form of soft, nonwoven fibrous webs. Such a web is produced from an aqueous fiber-forming polymer solution by first forming the polymer solution into filaments which are contacted with a primary air stream having a velocity sufficient to attenuate the filaments. The attenuated filaments are contacted in a fiber-forming zone with a secondary air stream having a velocity effective to further attenuate and to fragment the filaments into fibers and to transport the fibers to a web-forming zone. The fibers are collected in reticulated web form in the web-forming zone, and the web is cured. Hydrophilic thermosetting and thermoplastic polymer compositions of all types are stated to be useful in the foregoing process. However, such process allegedly has particular applicability when the polymer composition comprises a blend of (1) a copolymer of at least one alpha, beta-unsaturated carboxylic monomer and at least one monomer copolymerizable therewith, and (2) a crosslinking agent comprising hydroxyl or heterocyclic carbonate groups.
European Published Patent Application No. 0 176 316 A2 describes a nonwoven fabric of water-soluble resin fibers. The fabric consists of water-soluble resin fine fibers having a mean fiber diameter of 30 .mu.m or less and a basis weight of 5 to 500 g/m.sup.2. The fabric is produced by extruding an aqueous solution comprising a water-soluble resin or a melt of a water-soluble resin plasticized with water through nozzles, stretching the extruded material to form fibers by a high speed gas flow, heating the fibers to evaporate the water in the fibers, and then collecting the fibers. The water-soluble resins which can be used are stated to include poly(vinyl alcohol), although the application clearly is directed primarily to the use of pullulan, a natural glucan. The high speed gas flow typically consists of air at a temperature of from 20.degree. C. to 60.degree. C. and having a linear velocity of, e.g., 10 to 1,000 m/sec. Drying of the fibers is accomplished by banks of infrared heaters located on both sides of and parallel to the fiber stream.
Other methods of forming fibrous webs or products from a solution of a polymer (or molten polymer) are described in, by way of illustration only, U.S. Pat. Nos. 2,357,392 to Francis, Jr.; 2,411,660 to Manning; 2,464,301 to Francis, Jr.; 2,483,405 to Francis, Jr.; 2,483,406 to Francis, Jr.; 2,988,469 to Watson; 3,110,642 to Harrington et al.; and 4,234,652 to Vanoni et al. Such methods typically produce very short fibers and, consequently, differ significantly from the more traditional meltblowing or spunbonding processes which commonly are used to prepared nonwoven webs from molten thermoplastic polymers. See also U.S. Pat. Nos. 2,571,457 to Ladisch; 3,016,599 to Perry, Jr.; 3,073,735 to Till et al.; 3,379,811 to Hartmann et al.; 3,429,953 to Crompton; 3,535,415 to Ultee; 3,689,342 to Vogt et al.; 3,752,613 to Vogt et al.; 3,770,856 to Ueki et al.; 3,772,417 to Vogt; 3,801,400 to Vogt et al.; 3,914,354 to Ueki et al.; 4,011,067 to Carey, Jr.; 4,042,740 to Krueger; 4,043,331 to Martin et al.; 4,103,058 to Humlicek; 4,104,340 to Ward; 4,118,531 to Hauser; 4,137,379 to Schmidt et al.; 4,429,001 to Kolpin et al.; 4,726,901 to Pall et al.; 4,741,941 to Englebert et al.; and 4,755,178 to Insley et al.; British Patent No. 827,644; and Japanese Patent No. 90/2,970B.
The use of steam in fiber-forming processes is illustrated by, for example, U.S. Pat. Nos. 2,571,457, supra; 3,110,642 supra; 3,379,811, supra; 4,211,737 to Di Drusco et al.; 4,355,081 to Kinsley, Jr.; and 4,468,241 to Breidenthal et al. Note that (1) U.S. Pat. No. 4,808,367 to Homma et al. describes a water-containing polymeric composition which can be extruded under conditions such that the flashing of water is prevented, (2) U.S. Pat. No. 4,734,227 to Smith describes the formation of fibers using a supercritical fluid solution, and (3) U.S. Pat. No. 4,174,417 to Rydell relates to the spraying of water-imbibed gelled fibers to form webs.
Traditional meltblowing processes are illustrated by, for example, U.S. Pat. Nos. 3,016,599 to Perry, Jr.; 3,704,198 to Prentice; 3,755,527 to Keller et al.; 3,849,241 to Butin et al.; 3,978,185 to Butin et al.; 4,295,809 to Mikami et al.; 4,375,446 to Fujii et al.; and 4,663,220 to Wisneski et al. See, also, V. A. Wente, "Superfine Thermoplastic Fibers", Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346 (1956); V. A. Wente et al., "Manufacture of Superfine Organic Fibers", Navy Research Laboratory, Washington, D.C., NRL Report 4364 (111437), dated May 25, 1954, United States Department of Commerce, Office of Technical Services; and Robert R. Butin and Dwight T. Lohkamp, "Melt Blowing--A One-Step Web Process for New Nonwoven Products", Journal of the Technical Association of the Pulp and Paper Industry, Vol. 56, No. 4, pp. 74-77 (1973).
Coforming references (i.e., references disclosing a meltblowing process in which fibers or particles are comingled with the meltblown fibers as they are formed) include U.S. Pat. Nos. 4,100,324 to Anderson et al.; 4,118,531 to Hauser; 4,238,175 to Fujii et al.; and 4,442,062 to Fujii et al.
Finally, spunbonding references include, among others, U.S. Pat. Nos. 3,341,394 to Kinney; 3,655,862 to Dorschner et al.; 3,692,618 to Dorschner et al.; 3,705,068 to Dobo et al.; 3,802,817 to Matsuki et al.; 3,853,651 to Porte; 4,064,605 to Akiyama et al.; 4,091,140 to Harmon; 4,100,319 to Schwartz; 4,340,563 to Appel et al.; 4,405,297 to Appel et al.; 4,434,204 to Hartman et al.; 4,627,811 to Greiser et al.; and 4,644,045 to Fowells.
Although many advances have been made over the years in the formation of fibers and nonwoven webs from natural and synthetic polymers, there still is a need for improvements. This is particularly true in the formation of nonwoven webs from poly(vinyl alcohol), where known processes have a pronounced tendency to give webs having one or more of a number of deficiencies. Such deficiencies include poor web formation, namely, significant basis weight variation on a relatively small scale defined hereinafter; significant amounts of shot, i.e., small particles of solidified polymer separate from or associated with the fibers making up the web; and highly variable fiber diameters.